1. Field of the Invention
The present invention relates to the constructions of an inductor and a circuit thereof. In particular, the present invention relates to a current sensing inductor and a circuit thereof.
2. Description of Related Art
Inductors are commonly used in switched mode power conversion circuits for energy storage and for current and voltage filtering. In those applications, the instantaneous current flowing through inductor is a critical parameter for proper control circuit operation. Therefore, a variety of methods for measuring inductor current have been proposed.
Reference is made to FIG. 1, which shows a circuit diagram of a current sensing circuit of the prior art, wherein the circuit shows the output portion of the buck or forward type converter. As shown in FIG. 1, the current sensing circuit 1 includes an inductor 11, a current sensing resistor 12, and a sensing amplifier 15. The inductor 11 includes an inductive winding 111 and a winding resistance 113. An inductor current flowing through the inductor 11 is transferred to a capacitor 13 via a current sensing resistor 12. When inductor current flows, it flows through the current sensing resistor 12 and develops a small voltage drop across the current sensing resistor 12. The sensing amplifier 15 detects this small voltage drop and amplifies it for the use of a control circuit (not shown in the figure). Although the voltage drop across the current sensing resistor 12 is low, the power loss can be large when the inductor current is high. For example, when the voltage drop is 50 mV but the inductor current is 20 A, a significant power loss of 1 W is resulted.
Reference is made to FIG. 2, which shows the circuit diagram of another current sensing circuit of the prior art. As shown in FIG. 2, the current sensing circuit 2 includes an inductor 21, a current sensing transformer 22, a sensing amplifier 25, a rectifier diode 27, and a current sensing resistor 29. The inductor 21 includes an inductive winding 211 and a winding resistance 213. An inductor current flowing through the inductor 21 is transferred to a capacitor 23 via a current sensing transformer 22. When inductor current flows, it flows through the current sensing transformer 22 and induces a current into the current sensing resistor 29 via the rectifier diode 27. A voltage drop is developed across the current sensing resistor 29. The sensing amplifier 15 detects this voltage drop and amplifies it for the use of a control circuit (not shown in the figure).
Compared with FIG. 1, the current sensing circuit in FIG. 2 uses the turn-ratio of the current sensing transformer 22 to multiply the voltage drop across the current sensing resistor 29. Therefore, the voltage drop across the primary winding 221 of the current sensing transformer 22 is very small and hence the power loss problem is mitigated. However, the current sensing circuit in FIG. 2 is not suitable for continuous-inductor-current-mode operation. Because DC component within the inductor current will magnetically saturate the current sensing transformer 22 and make it useless. With the circuit operating in discontinuous-inductor-current-mode, the rectifier diode 27 allows the current sensing transformer 22 to magnetically reset whenever the inductor current reverses of interrupts. Even though, the accuracy of this sensing circuit is suffered from magnetising current within the current sensing transformer 22 when the inductor current flows uninterrupted for too long a period.
Reference is made to FIG. 3, which shows a circuit diagram of another current sensing,circuit of the prior art. As shown in FIG. 3, the current sensing circuit 3 includes an inductor 31, a low-pass filter 32, and a sensing amplifier 35. The inductor 31 includes an inductive winding 311 and a winding resistance 313. The winding resistance 313 is used as a current sensing resistor. An inductor current flowing through the inductor 31 is transferred to a capacitor 33. The low-pass filter 32 includes a series resistor 321 and a shunt capacitor 323 for sensing the small voltage drop across the winding resistance 313. The voltage across the whole inductor 31 is transferred via the low-pass filter 32 to the sensing amplifier 35. The sensing amplifier amplifies its input for the use of a control circuit (not shown in the figure).
The low-pass filter 32 is necessary for attenuating the large ripple voltage existing across the inductive winding 311. Without a strong low-pass filter, the relatively weak voltage drop across the winding resistance 313 won't be detectable. This sensing circuit eliminates the power loss associated with an explicit current sensing resistor. But the strong low-pass filter introduces heavy delay to the detection response time. That makes control loop compensation difficult. Also, the value of winding resistance 313 is often sensitive to temperature variations. That makes current sensing very inaccurate.